The archosaurs are characterized by numerous synapomorphies that lend strong
support to the hypothesis that they form a monophyletic group (clade) exclusive of
other Reptilia. First of all, the "stem archosaurs" (properly termed
Archosauromorpha), including Champsosauridae and Euparkeria,
have a calcaneal tuber. This is a bony process projecting
posteriorly from the ankle joint that serves as an attachment point for some of the lower
leg flexor muscles. If you feel your heel, the bone that forms it is your
calcaneal tuber. This is an example of convergent evolution;
synapsids and
archosaurs evolved these features independently.

Archosaur skull: The archosaur skull includes two new fenestra
(skull openings). The first of these is the mandibular fenestra (6), an
opening through the jawbone. The other is the antorbital fenestra (2),
a term that means "in front of the opening for the eye."

Later archosauromorphs, like Euparkeria and the true archosaurs, evolved
several key diagnostic features. The picture at right shows the two new skull
openings, the mandibular fenestra and the antorbital fenestra, that
appeared in archosauromorphs close to the Archosauria. Another new feature that
appeared around this point was the presence of serrated (saw-edged) teeth set in
sockets (a condition called thecodonty). Also, a large process on the shaft
of the femur, the fourth trochanter, served as the attachment
point for major tail muscles, the caudofemoralis group of thigh retracting
muscles. Finally, ancestral archosaurs had a double row of bony plates (called
scutes, or osteoderms) running along the backbone.

True archosaurs (the Archosauria proper), including the major stem groups
Pseudosuchia (the crocodilians and their relatives) and the Ornithosuchia (the birds
and their relatives) evolved yet more new features. The fifth toe in the foot,
homologous with your "pinky toe" was reduced in size. In earlier vertebrates, the
palate (roof of the mouth) bore at least one row of accessory teeth, but the
archosaurs appear to have lost this feature, as did many other lineages of tetrapods.
Finally, the two main lineages of archosaurs each have a specialized ankle joint that
changed differently in each group, as shown below.

Archosaur ankles: The two major groups of archosaurs are distinguished
by differences in the joint of their ankle. The crocodiles and their relatives
(Pseudosuchia) have a crurotarsal ankle joint (above at left),
while dinosaurs and pterosaurs (Ornithosuchia) have a mesotarsal
ankle joint (above at right). The red line in each image shows the plane of
the ankle hinge. (T=tibia, F=fibula, A=astragalus, C=calcaneum)

The Pseudosuchia have what is called a crurotarsal, or "crocodile-normal"
ankle. This is a very flexible arrangement in which the astragalus (medial proximal
ankle bone) bears a peg that fits into a socket in the calcaneum (lateral proximal
ankle bone). Ankle rotation then occurs between these two bones, permitting both
a somewhat erect stance (like the crocodilian "high walk") where the hindlimb is
held closer to the midline of the body, and a more sprawling stance like that of
earlier tetrapods. The result of this is that the pseudosuchians can move in two
different ways; walking with an erect or sprawling posture.

The Ornithosuchia have a mesotarsal ankle, which is a simple hinge joint
between the lower leg and astragalus and calcaneum, and the distal ankle bones.
This restricts the posture to a more erect orientation, so the gait can be called
parasagittal  the limbs move parallel to the vertebral column, and are
held relatively vertical. Birds and most mammals have this parasagittal gait;
birds inherited it from their
dinosaurian ancestors, while
mammals evolved it
independently. The advantage of a parasagittal gait might be that it improves
maneuverability/agility. A concordant disadvantage would be that it reduces
stability. It's easier to tip over an erect cow than it is to tip over a more
sprawling crocodile of similar size; the crocodile has a wider base of support, and thus can be
said to be more stable. However, the relationship between stance and stability
is still not completely understood, so it's difficult to say exactly what advantages an
erect stance has. Another proposed advantage of an erect stance and parasagittal
gait was that it was more efficient, but this has not held up to experimental analysis  erect animals move about as efficiently as similarly-sized sprawling ones.

It is difficult to say what sorts of soft tissues extinct archosaurs really had, but the fact that
both crocodilians and birds have a four-chambered heart lends support to the notion that this is a
trait inherited from a common ancestor (and another convergence with mammals).
Birds and crocodiles also share expanded pneumatic sinuses in their skulls  and elsewhere in
bird skeletons  which can also be seen in fossil archosaurs, such as the
hadrosaurs,
so the expansion of these craniofacial air sinuses is another trait shared
by archosaurs. The sinuses appear to serve as skull-remodeling agents,
reshaping the bones of the skull in response to stresses and other influences.